Catalytic converter for automotive pollution control, and process for making catalytic converter

ABSTRACT

A catalytic converter for cleaning exhaust gas includes a heat-resistant support, and a coating formed on the support. The coating includes at least one kind of catalytically active substance and a zirconium oxide. The zirconium oxide has a pre-aging specific surface area I and a post-aging specific surface area A, wherein the aging is performed in an atmosphere of 1,000° C. for 5 hours, and wherein A/I≧0.4 and I≧40m 2 /g.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a catalytic converter foreffectively cleaning the exhaust gas of an automotive internalcombustion engine by removal of nitrogen oxide (NO_(x)), carbon monoxide(CO) and hydrocarbons (HC). The present invention also relates to aprocess for making such a catalytic converter.

[0003] 2. Description of the Related Art

[0004] As is well known, the exhaust gas of an automotive internalcombustion engine inevitably contains harmful substances such as NO_(x),CO and HC. In recent years, particularly, the restrictions on exhaustgas cleaning are increasingly strict for environmental protection.

[0005] A so-called three-way catalytic converter has been most widelyused for removing the above-described harmful substances. Typically, athree-way catalytic converter includes a honeycomb support made of aheat-resistant material such as cordierite, and a wash-coat formed onthe surfaces of the respective cells of the honeycomb support. Thewash-coat contains a catalytically active substance such as Pt, Pdand/or Rh, and carrier oxide powder such as zirconium oxide powder forsupporting the catalytically active substance. The catalytically activesubstance reduces NO_(x) to N₂ while oxidizing CO and HC to CO₂ and H₂O,respectively.

[0006] However, it has been found that the grains or particles ofzirconium oxide powder (as the carrier oxide) grows due to sintering athigh temperature. Such grain growth of zirconium oxide results in adecrease of surface area, consequently lowering the catalytic activityof the catalytic converter as a whole. Particularly, if the catalyticconverter is mounted near the engine, it may be frequently subjected toan extremely high temperature of no less than 900° C. (or sometimes evenhigher than 1,000° C.), which prompts the grain growth of zirconiumoxide powder.

[0007] A conventional counter measure against such a problem is to lowerthe heat-treating temperature at the time of preparing zirconium oxidepowder and/or at the time of coating the prepared zirconium oxide powderonto the honeycomb support for increasing the initial specific surfacearea of the zirconium oxide powder. It is true that such a countermeasure allows for a subsequent decrease of the specific surface area,thereby prolonging the time needed until the specific surface area ofthe zirconium oxide powder drops to a certain level. On the other hand,however, an initial increase of the specific surface area results in agreater extent of sintering (i.e., a greater decrease of the specificsurface area) upon lapse of a relatively long time, thereby causing thecatalytically active substance (Pt, Rh and/or Pd) to be buried in thesintered zirconium oxide powder. As a result, the catalytic activity ofthe catalytic converter drops remarkably in the long run.

DISCLOSURE OF THE INVENTION

[0008] It is, therefore, an object of the present invention to provide acatalytic converter for cleaning exhaust gas which is capable ofretaining its catalytic activity for a long time even under severeoperating conditions above 900° C. for example.

[0009] Another object of the present invention is to provide a processfor advantageously making such a catalytic converter.

[0010] According to one aspect of the present invention, a catalyticconverter for cleaning exhaust gas comprises a heat-resistant support;and a coating formed on the support, the coating including at least onekind of catalytically active substance and a zirconium oxide; whereinthe zirconium oxide having a pre-aging specific surface area I and apost-aging specific surface area A, the aging being performed in anatmosphere of 1,000° C. for 5 hours; and wherein A/I≧0.4 and I≧40m²/g.

[0011] The zirconium oxide incorporated in the coating of the catalyticconverter described above exhibits a relatively small decrease ofspecific surface area (i.e., a relatively high A/I value) even after thehigh temperature aging (1,000° C., 5 hours). Therefore, even if thecatalytic converter is repetitively subjected to a high temperature ofno less than 900° C., the zirconium oxide is subsequently sintered onlyto a limited extent. As a result, the catalytic activity of thecatalytic converter can be maintained for a longer time than isconventionally possible.

[0012] The zirconium oxide, which experiences a relatively smalldecrease of specific surface area, may be prepared by suitably adjustingthe composition of the zirconium oxide or by suitably adjusting theconditions for making the zirconium oxide.

[0013] More specifically, the zirconium oxide may be a zirconium complexoxide represented by the following formula,

Zr_(1−(x+y))Ce_(x)R_(y)Oxide

[0014] where R represents a rare earth element other than Ce or analkaline earth metal, and where the zirconium complex oxide meets0.12≦x≦0.25 and 0.02≦y≦0.15 in this formula.

[0015] Examples of rare earth elements “R” other than Ce include Sc, Y,La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Of theseexamples, La and Nd are preferred. Examples of alkaline earth metalsinclude Be, Mg, Ca, Sr and Ba.

[0016] Alternatively, the zirconium oxide may be subjected to apreliminary aging or baking step at a high temperature for positivelycausing grain growth (sintering). Such preliminary aging or sinteringrestrains or limits subsequent grain growth (a decrease of specificsurface area) under high temperature operating conditions, therebyprolonging the service life of the catalytic converter.

[0017] Typically, the catalytically active substance contained in thecoating may be a precious metal such as Ru, Rh, Pd, Ag, Os, Ir, Pt andAu. Preferably, however, the catalytically active substance may beselected from a group consisting of Pt, Rh and Pd. Each of these activesubstances may be used alone or in combination with another.

[0018] The coating may also contain at least one heat-resistantinorganic oxide selected from a group consisting of alumina (Al₂O₃),silica (SiO₂), titania (TiO₂) and magnesia (MgO). Particularly useful isactivated alumina. Further, the coating may further comprise anoxygen-storing oxide such as cerium complex oxide.

[0019] The catalytically active substance may be supported selectivelyon the particles of the zirconium oxide or the heat-resistant inorganicoxide before the zirconium oxide or the inorganic oxide is coated on theheat-resistant support. Alternatively, the catalytically activesubstance may be coated on the heat-resistant support at the same timewhen the zirconium oxide (and optionally the inorganic oxide) is coatedon the support. Further, the catalytically active substance may besupported at the surface of the coating after the zirconium oxide (andoptionally the inorganic oxide) is coated first on the heat-resistantsupport.

[0020] The heat-resistant support, which may be made of cordierite,mullite, α-alumina or a metal (e.g. stainless steel), should preferablyhave a honeycomb structure. In this case, the coating is formed in eachcell of the honeycomb structure.

[0021] The zirconium complex oxide having the above formula may beprepared by using known techniques such as coprecipitation process oralkoxide process.

[0022] The coprecipitation process includes the steps of preparing amixture solution which contains respective salts of Ce, Zr and otherelement (a rare earth element other than Ce or an alkaline earth metal)in a predetermined stoichiometric ratio, then adding an aqueous alkalinesolution or an organic acid to the salt solution for causing therespective salts to coprecipitate, and thereafter heat-treating theresulting coprecipitate for oxidization to provide a target complexoxide.

[0023] Examples of starting salts include sulfates, nitrates,hydrochlorides, phosphates, acetates, oxalates, oxychloride, oxynitrate,oxysulfate and oxyacetate. Examples of aqueous alkaline solutionsinclude an aqueous solution of sodium carbonate, aqueous ammonia and anaqueous solution of ammonium carbonate. Examples of organic acidsinclude oxalic acid and citric acid.

[0024] The heat treatment in the coprecipitation process includes aheat-drying step for drying the coprecipitate at about 50˜200° C. forabout 1˜48 hours after filtration, and a baking step for baking thecoprecipitate at about 350˜1,000° C. (preferably about 400˜700° C.) forabout 1˜12 hours. During the baking step, the baking conditions (thebaking temperature and the baking period) should be selected dependingon the composition of the zirconium complex oxide so that at least partof the complex oxide is in the form of solid solution.

[0025] The alkoxide process includes the steps of preparing an alkoxidemixture solution which contains Ce, Zr and other element (a rare earthelement other than Ce or an alkaline earth metal) in a predeterminedstoichiometric ratio, then adding a deionized water to the alkoxidemixture solution for causing Ce, Zr and the other element to hydrolyze,and thereafter heat-treating the resulting hydrolysate to provide atarget complex oxide.

[0026] Examples of alkoxides usable for preparing the alkoxide mixturesolution include methoxides, ethoxides, propoxides and butoxides.Instead, ethylene oxide addition salts of each of the elements are alsousable.

[0027] The heat treatment in the alkoxide process may be performed inthe same way as that in the coprecipitation process.

[0028] A precious metal such as Pt, Rh or Pd as the catalytically activesubstance may be supported on the zirconium oxide or the heat-resistantinorganic oxide (other than the zirconium oxide) by using knowntechniques. For instance, a solution containing a respective salt (e.g.1-20 wt %) of Pt (and/or Rh and/or Pd) is first prepared, the zirconiumoxide or the other heat-resistant inorganic oxide is then impregnatedwith the salt-containing solution, and thereafter the oxide isheat-treated. Examples of salts usable for this purpose include nitrate,dinitro diammine nitrate, and chloride. The heat-treatment, which isperformed after impregnation and filtration, may include drying theoxide by heating at about 50˜200° C. for about 1˜48 hours and thereafterbaking the complex oxide at about 350˜1,000° C. for about 1˜12 hours.

[0029] Alternatively, a precious metal may be supported on the zirconiumoxide or the other heat-resistant inorganic oxide at the time ofperforming the coprecipitation process or the alkoxide process by addinga salt solution of the precious metal to the mixture salt solution orthe alkoxide mixture solution.

[0030] The coating may be formed by mixing the zirconium oxide or theother heat-resistant inorganic oxide with distilled water to prepare anaqueous slurry, then depositing the slurry on the heat-resistantsupport, and finally drying the support in an electric oven. Thecatalytically active substance may be supported selectively on thezirconium oxide or the heat-resistant inorganic oxide before thezirconium oxide or the inorganic oxide is coated on the heat-resistantsupport. Alternatively, the catalytically active substance may be coatedon the heat-resistant support at the same time when the zirconium oxide(and optionally the other inorganic oxide) is coated on the support.Further, the catalytically active substance may be supported at thesurface of the coating after the zirconium oxide (and optionally theother inorganic oxide) is coated first on the heat-resistant support.

[0031] According to a second aspect of the present invention, a processfor making a catalytic converter for cleaning exhaust gas is providedwhich comprises the steps of: performing preliminary baking of azirconium oxide for causing a decrease in specific surface area of thezirconium oxide; and coating the preliminarily baked zirconium oxide ona heat-resistant support together with at least one kind ofcatalytically active substance.

[0032] As previously described, since the zirconium oxide is subjectedto the preliminary baking before being coated on the heat-resistantsupport, the specific surface area of the zirconium oxide decreases to acertain level in advance and therefore undergoes a subsequent decreaseof specific surface area only to a limited extent. As a result, theservice life of the catalytic converter is correspondingly prolonged.

[0033] Preferably, the preliminary baking step may be performed at atemperature of not lower than 700° C.

[0034] Again, the zirconium oxide may be a zirconium complex oxiderepresented by the following formula,

Zr_(1−(x+y))Ce_(x)R_(y)Oxide

[0035] where R represents a rare earth element other than Ce or analkaline earth metal, the zirconium complex oxide meeting 0.12≦x≦0.25and 0.02≦y≦0.15 in said formula. Further, the catalytically activesubstance may be selected from a group consisting of Pt, Rh and Pd.

[0036] Other features and advantages of the present invention will beapparent from the following detailed description of the preferredembodiments given with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Next, various examples of the present invention will be describedtogether with comparative examples. However, it should be appreciatedthat the present invention is in no way limited by these examples.

EXAMPLE 1

[0038] In Example 1, zirconium complex oxide having a composition ofZr_(0.8)Ce_(0.16)Nd_(0.02)La_(0.02)Oxide/2 wt % Pt/2 wt % Rh wasprepared and determined for its specific surface area and catalyticactivity before and after high-temperature redox aging, respectively.Here, the notation “/2 wt % Pt/2 wt % Rh” represents that 100 parts byweight of zirconium complex oxide (not supporting any precious metal)supports 2 parts by weight of Pt and 2 parts by weight of Rh.

[0039] (Preparation of Zirconium Complex Oxide)

[0040] The zirconium complex oxide having the above-noted compositionwas prepared by the so-called alkoxide process. Specifically, analkoxide mixture solution was first prepared by dissolving, in 200 cm³of toluene, 61 g (0.136 mol) of zirconium ethoxyethylate, 13.5 g (0.0272mol) of cerium ethoxyethylate, 1.4 g (0.0034 mol) of neodymiumethoxyethylate, and 1.4 g (0.0034 mol) of lanthanum ethoxyethylate.Then, the alkoxide mixture solution was gradually dripped into 600 cm³of deionized water in about 10 minutes for causing hydrolysis of thealkoxide mixture. Then, the toluene and water content of the alkoxidemixture solution was removed by vaporization. Then, the remaininghydrolysate (precursor) was dried by ventilation at 60° C. for 24 hours.Then, the resulting zirconium complex oxide was baked in an electricoven at 800° C. for 1 hour for causing preliminary grain growth(preliminary sintering), thereby providing powder of the targetCe—Zr—Nd—La complex oxide.

[0041] Further, the zirconium complex oxide powder was impregnated withan aqueous solution of dinitro diammineplatinum nitrate and rhodiumnitrate. The thus impregnated powder was first dried at 60° C. for 24hours and then baked at 600° C. for 3 hours. As a result, the zirconiumcomplex oxide was made to support 2 parts by weight of Pt and 2 parts byweight of Rh relative to 100 parts by weight of the complex oxide.

[0042] (High-Temperature Redox Aging)

[0043] The Pt- and Rh-supporting zirconium complex oxide thus obtainedwas subjected to high-temperature redox aging by cyclically placing thezirconium complex oxide in three different atmospheres each held at ahigh temperature of about 1,000° C. More specifically, a cycle of 30minutes was repeated ten times for a total time of 5 hours, in whichcycle the oxygen-storing oxide was placed in an inert atmosphere for 5minutes, then in an oxidizing atmosphere for 10 minutes, again in theinert atmosphere for 5 minutes, and finally in a reducing atmosphere for10 minutes. The respective composition of the oxidizing atmosphere, theinert atmosphere and the reducing atmosphere used here is listed inTable 1 below. During this test, each of the three different atmosphereswas supplied at a flow rate of 300 dm³/hr and maintained at atemperature of about 1,000° C. by the inclusion of high-temperature H₂Ovapor. TABLE 1 Components Oxidizing Inert Reducing H₂ — — 0.5 vol % CO —— 1.5 vol % O₂ 1.0 vol % — — CO₂ 8.0 vol % 8.0 vol % 8.0 vol % H₂O 10vol % 10 vol % 10 vol % N₂ 81 vol % 82 vol % 80 vol %

[0044] (Determination of Specific Surface Area)

[0045] The specific surface area of the zirconium complex oxide wasdetermined before and after the high-temperature redox aging,respectively, in accordance with the BET adsorption isotherm methodwhich itself is well known.

[0046] (Determination of Catalytic Activity)

[0047] The catalytic activity of the Pt- and Rh-supporting zirconiumcomplex oxide was evaluated by determining the CO—NO_(x) cross pointremoval before and after the high-temperature redox aging, respectively.The evaluation of the catalytic activity by calculating the value(quotient) of (pre-aging removal)/(post-aging removal). The “CO—NO_(x)cross point removal” as used herein means the point (removal inpercentage) where the CO removal and the NO_(x) removal coincide whilethe exhaust gas being cleaned changes gradually in composition from afuel-rich state to a fuel-lean state.

[0048] (Results)

[0049] The results of the surface area determination and the catalyticactivity determination in Example 1 are shown in Table 2 below togetherwith those for Examples 2˜3 and Comparison 3 to be describedhereinafter. TABLE 2 Specific Catalytic Baking Surface Area ActivityTemp. (° C.) *Pre(m²/g) **Post(m²/g) Pre/Post Pre/Post Ex. 1 800 70 400.57 0.95 Ex. 2 900 56 44 0.78 0.97 Ex. 3 1000  47 46 0.98 0.99 Com. 1400 150  25 0.17 0.60

EXAMPLES 2˜3 AND COMPARISON 1

[0050] In Examples 2˜3 and Comparison 1, zirconium complex oxide havingthe same composition as that of Example 1 was prepared in the samemanner except that the zirconium complex oxide was subjected topreliminary baking at respective temperatures of 900° C. (Example2),1,000° C. (Example3) and 400° C. (Comparison 1). Then, the zirconiumcomplex oxide was determined for its specific surface area and catalyticactivity before and after high-temperature redox aging in the samemanner as in Example 1.

[0051] The results of the surface area determination and the catalyticactivity determination in these examples are also shown in Table 2above.

EXAMPLES 4˜7 AND COMPARISON 2

[0052] In Examples 4˜7 and Comparison 2, zirconium complex oxide havinga composition of Zr_(0.75)Ce_(0.2)Y_(0.05)Oxide/2 wt % Pt/2 wt % Rh wasprepared and determined for its specific surface area and catalyticactivity before and after high-temperature redox aging, respectively, inthe manner similar to Example 1. However, in these examples, thepreliminary baking was performed at respective temperatures of 700° C.(Example 4), 800° C. (Example 5), 900° C. (Example 6), 1,000° C.(Example 7) and 400° C. (Comparison 2).

[0053] The results of the surface area determination and the catalyticactivity determination in these examples are shown in Table 3 below.TABLE 3 Specific Surface Area Baking Temp. *Pre **Post CatalyticActivity (° C.) (m²/g) (m²/g) Pre/Post Pre/Post Ex. 4 700 110  45 0.410.92 Ex. 5 800 75 50 0.67 0.95 Ex. 6 900 60 51 0.85 0.97 Ex. 7 1000  5554 0.98 0.99 Com. 2 400 155  22 0.14 0.55

Evaluation of Examples 1˜7 and Comparisons 1˜2

[0054] From Tables 2 and 3, it is observed that the zirconium complexoxide, when subjected to preliminary baking at a temperature of no lessthan 700° C., suffered a lesser decrease of specific surface area thanwhen baked at a temperature of 400° C. (which was a normal bakingtemperature). Further, it is also observed that the post-aging specificsurface area of the zirconium complex oxide was higher whenpreliminarily baked at a temperature of no less than 700° C. than whenbaked at a temperature of. This is why the Pt- and Rh-supportingzirconium complex oxide retained a high catalytic activity even afterthe high-temperature redox aging (see the last column in Tables 2 and3). By contrast, the zirconium complex oxide, when preliminary baked ata low temperature of 400° C., exhibited a higher CO—NO_(x) cross pointremoval at an initial stage but soon lost its catalytic activity afterthe high-temperature redox aging.

[0055] Thus, it is concluded that the zirconium oxide (or zirconiumcomplex oxide) according to the present invention provides a relativelyhigh catalytic activity for a long time.

1. A catalytic converter for cleaning exhaust gas comprising: aheat-resistant support; and a coating formed on the support, the coatingincluding at least one kind of catalytically active substance and azirconium oxide; wherein the zirconium oxide having a pre-aging specificsurface area I and a post-aging specific surface area A, the aging beingperformed in an atmosphere of 1,000° C. for 5 hours; and wherein A/I≧0.4and I≧40m²/g.
 2. The catalytic converter according to claim 1, whereinthe zirconium oxide is a zirconium complex oxide represented by thefollowing formula, Zr_(1−(x+y))Ce_(x)R_(y)Oxide where R represents arare earth element other than Ce or an alkaline earth metal, thezirconium complex oxide meeting 0.12≦x≦0.25 and 0.02≦y≦0.15 in saidformula.
 3. The catalytic converter according to claim 1, wherein thecatalytically active substance is selected from a group consisting ofPt, Rh and Pd.
 4. The catalytic converter according to claim 1, whereinthe coating further comprises an oxygen-storing oxide.
 5. The catalyticconverter according to claim 4, wherein the oxygen-storing oxide is acerium complex oxide.
 6. The catalytic converter according to claim 1,wherein the coating further comprises at least one heat-resistantinorganic oxide selected from a group consisting of alumina, silica,titania and magnesia.
 7. The catalytic converter according to claim 1,wherein the heat-resistant support has a honeycomb structure.
 8. Aprocess for making a catalytic converter for cleaning exhaust gas,comprising the steps of: performing preliminary baking of a zirconiumoxide for causing a decrease in specific surface area of the zirconiumoxide; and coating the preliminarily baked zirconium oxide on aheat-resistant support together with at least one kind of catalyticallyactive substance.
 9. The process according to claim 8, wherein thepreliminary baking step is performed at a temperature of not lower than700° C.
 10. The process according to claim 8, wherein the preliminarilybaked zirconium complex oxide is first treated to support thecatalytically active substance and then coated on the heat-resistantsupport.
 11. The process according to claim 8, wherein the zirconiumoxide is a zirconium complex oxide represented by the following formula,Zr_(1−(x+y))Ce_(x)R_(y)Oxide where R represents a rare earth elementother than Ce or an alkaline earth metal, the zirconium complex oxidemeeting 0.12≦x≦0.25 and 0.02≦y≦0.15 in said formula.
 12. The processaccording to claim 8, wherein the catalytically active substance isselected from a group consisting of Pt, Rh and Pd.